This invention pertains to metal vapor lasers generally, and in particular to the plasma tube housing of the laser. The disclosed plasma tube is especially notable for its ability to recycle condensed metal vapor.
Plasma tubes are the components of a laser that house the lasing medium. In metal vapor lasers, this medium is a metal that has been vaporized. Although any metal will, in theory, support lasing, some are much more commonly used. To date, copper has been the most popular choice. Other common lasing media include lead and cadmium. When excited, the lasing medium can emit coherent laser light at a particular wavelength. Copper vapor lasers, for example, typically produce light in the green region of the visible spectrum. Lead vapor lasers generate light in the red region.
Conventional plasma tubes are simply cylindrical tubes. A solid piece of the lasing media is placed at the center of the plasma tube before it is put into operation. An electric discharge along the length of the plasma tube vaporizes and excites the metal atoms. During lasing, the center of the plasma tube becomes the hottest region and it is there that the metal most easily vaporizes. However, the metal vapor continuously diffuses throughout the entire volume of the plasma tube where the metal vapor cools and often condenses. The heat generated by the discharge causes most of the condensed metal drops to ultimately revaporize. However, since the ends of the tube are cooler than the center of the tube, some of the metal condensing at the ends is not revaporized and thus tends to accumulate there. Since the accumulated metal cannot readily be used to support lasing, this "migration" of the metal towards the ends of the plasma tube shortens the useful life of the laser before the lasing media must be recharged.
This accumulation also degrades the plasma tube performance in other ways. For example, the insulation of the plasma tube is compromised, necessitating an increased power input. Also, the accumulation of deposited material gradually decreases the inside diameter of the plasma tube end portions. Since a substantial percentage of the lasing energy generated in high energy lasers accumulates very close to the walls of the plasma tube, a decrease in tube diameter results in a corresponding decrease in laser output.
It has been recognized in the art that accumulation of the metal lasing media at the cool ends of plasma tubes creates some problems. A few preliminary attempts to rectify the problem have been made. Most of those disclosed are means for forcing the metal vapor back to the center of a plasma tube. For example, U.S. Pat. No. 4,710,938 discloses the use of anodes placed near the Brewster windows (at the ends of the laser tube) to "blow" the vapor back toward the center of the tube. However, this blowing actually causes some condensation on the end portions of the hollow tubes. This is the exact problem that the present invention seeks to avoid.
Wicks are also sometimes used to recycle condensed metal vapor. This approach is disclosed in U.S. Pat. Nos. 4,442,523, 3,654,567, and 4,247,830. The wicks are typically porous materials which draw condensed metal vapor back toward the center of the tube by capillary action. Wicks, however, require sophisticated materials that must have both carefully controlled porosity and resistance to the high temperatures of lasing.
Japanese Application No. 59-219210 discloses a plasma tube with symmetrically tapered ends designed to direct solidified metal from the ends of a plasma tube back toward the center. Although this device will permit recycling of metal condensed at the ends, it suffers from incomplete utilization of the laser periphery. As mentioned above, most of the power produced within the plasma tube resides along the edges of the tube itself. Since this region is partially occluded by the tapering about the entire perimeter, significant power reduction will result. Thus, lasers built in accordance with the teachings of Japanese Application No. 59-219210 suffer from an unnecessary loss of efficiency.